1. Field of the Invention:
The present invention relates to the stabilization of solutions of polyisocyanates in organic solvents against the formation of precipitates during storage.
2. Description of the Prior Art:
Because of their chemical and mechanical properties, polyurethane-based coatings are being widely used in this art as paints, lacquers and varnishes for various substrates such as metals, wood, concrete and polymers. In most cases, these polyurethane coatings are produced by the reaction, on a given substrate, of a polyhydroxylated compound (for example, polyols, polyhydroxylated polyethers or polyhydroxylated polyesters) with a compound containing a plurality of isocyanate groups, in the presence of adjuvants intended to promote the reaction and the hardening of the resultant film. The reactivity of the compounds containing free hydroxyl groups with the compounds containing free isocyanate groups necessitates that the contact between the two reactants take place only shortly prior to application, for example, in the mixing chamber of spraying equipment into which they are separately charged.
Thus, under these conditions, the formation of the polyurethane coating entails the use of two separate components, which accounts for such compositions being designated as two-component or two-part coating compositions (varnishes, paints or lacquers) in the art. In general, one of the two components contains the polyhydroxylated compound, to which various adjuvants may be added, such as pigments, thickeners, flow-improvers, stabilizers (for example, UV stabilizers) and, if appropriate, a polyaddition catalyst, and the other component is a solution of one or more compounds containing free isocyanate groups, in an organic solvent or a mixture of organic solvents which is intended to facilitate the use of the polyisocyanate component and its mixing with the polyhydroxylated component. The solvents used for this purpose are compounds which are liquid under conditions of normal pressure and temperature and which must be free from functional groups capable of reacting with isocyanate groups, and especially devoid of functional groups containing active Zerewitinoff hydrogen atoms, such as amino, hydroxyl, mercaptan and carboxyl groups. For these solutions to be stable, it is also important that the solvents be free from water, a condition which is especially difficult to meet on an industrial scale without having to resort to costly means for removing the last traces of water. Furthermore, whatever the efforts made to remove water, it is difficult to maintain the solvents in anhydrous state. In fact, water may be introduced adventitiously, notably by atmospheric water vapor in the case of hydrophilic solvents. It too has been found that the presence of trace amounts of water in organic solvents for polyisocyanates causes the formation of precipitates while they are in storage, which is harmful to their appearance and to their ultimate use and can spoil the appearance of the final coating. Consequently, serious need exists in this art for means to avoid, on the one hand, the use of costly processes for removing trace amounts of water from the solvents employed to produce organic solutions of polyisocyanates and, on the other hand, the formation of precipitates in these solutions containing trace amounts of water.